1. Field of the Invention
This invention relates to a push button switch, and more particularly to a push button switch suitable for use as signal input means in electronic desk-top calculators or the like.
2. Description of the Prior Art
Reed key switches have been widely used as signal input means in electronic desk-top calculators or the like. Such a reed key switch comprises a permanent magnet movable in response to movement of a key top, and a reed switch securely fixed to a member which is unaffected by movement of the key top (the reed switch comprises two reeds of magnetically conductive material led out of the opposite ends of a hermetically sealed cylindrical tube of glass or other magnetically non-conductive material, the free ends of the reeds disposed within the hermetically sealed tube in the center portion thereof being parallel and adjacent to each other).
The reed key switch is operable such that, when the key top is depressed, the permanent magnet is moved to cause the magnetic flux therefrom to pass through the reeds of the reed switch to attract the reeds into contact with each other. Thus, depression of the key top results in formation of a current flow path between the two reeds of the reed switch. As the key top is released from depression, the magnet is moved to liberate the reeds from its magnetic flux and disengage the reeds from each other, thus breaking the current flow path formed between the two reeds.
The reed key switch having the construction and performance as described above enjoys good durability and high reliability because the reeds acting as contacts are housed within the hermetically sealed container and driven not directly by mechanical means, whereas the reed key switch is relatively large-sized in proportion to the large size of the reed switch and of the magnet, and the resultant higher cost of manufacture limits the applications of such switch.
To overcome the above-noted disadvantages of such push button switches, conductive rubber keys are known which employ conductive rubber as bridge means (the conductive rubber is composed of rubber with a conductive material distributed therein). In such a key, conductive rubber is secured to the lower portion of a key top and a pair of contacts is secured to a based plate in opposed and predetermined spaced relationship with the conductive rubber. Depression of the key top brings the conductive rubber into engagement with the pair of contacts to thereby form a current flow path between the two contacts, and release of the pressure from the key top returns the key top to its original position and brings the conductive rubber out of engagement with the pair of contacts, thereby breaking the current flow path formed between the contacts.
This type of key switch is simple to construct and economical to manufacture and permits reduction in thickness or size, but the very short stroke of the key top and especially, the very short distance over which the key top is moved after the conductive rubber has engaged the pair of contacts, has not only made it difficult for the operator's finger to feel and judge whether the switch has assumed its ON position in response to the pressure imparted thereto by the finger, but also led to the possibility that any slightest vibration of the finger as it depresses the key top results in unstable opening-closing of the switch.